1. Field of the Invention
The present invention relates to a burner and to a method for operating a premix burner.
2. Brief Description of the Related Art
What are known as thermoacoustic fluctuations often occur in burners which supply liquid or gaseous fuel to a combustion chamber where the fuel burns at a flame front. This is true in particular if the burners are operated with high air ratio, for example, although not exclusively, in the case of what is known as the double-cone burner, as described EP-B1 0 321 809, which has been used with great success. Thermoacoustic vibrations of this nature also occur in the case of premix burners with a downstream mixing section, as described, for example, in EP-A2 0 704 657. In addition to the flow stability, mixing ratio fluctuations represent a primary reason for the occurrence of thermoacoustic instability of this nature. Flow instability waves which occur at the burner lead to the formation of turbulence (coherent structures), which can influence combustion and lead to periodic release of heat, with the associated fluctuations in pressure. The fluctuating air column in the burner leads to fluctuations in the mixing ratio, with the associated fluctuations in the release of heat. Moreover, fluctuations of this nature may also be caused by alternating flame front positions.
A further mechanism for exciting thermoacoustic oscillations is provided if, with a correct phase position (what is known as the Rayleigh criterion has to be satisfied, cf. below), local fluctuations in the release of heat are coupled with fluctuations in the mixing ratio via the fluctuating air column in the burner.
In burners of this type, there are often a plurality of fuel injection nozzles which are arranged in groups in order in this way to ensure stable combustion in different load ranges, for example special pilot nozzles for the lower load range. In this case, the flame position may shift significantly depending on the type of pilot control, and in such a case thermoacoustic fluctuations may also occur in transition regions as a result of a periodic change in the flame front positions.
These thermoacoustic oscillations pose a risk to any type of combustion application. They lead to high-amplitude pressure oscillations, to restrictions to the operating range and may also increase the emissions of pollutants. This applies in particular to combustion systems with little acoustic damping, such as for example annular combustion chambers with reverberant walls. In order to allow a high level of power conversion with regard to pulsations and emissions over a wide operating range, active control of the combustion oscillations may be required.
Coherent structures play a crucial role in mixing processes between air and fuel. The dynamics of these structures accordingly influence combustion and therefore the release of heat. Influencing the shear layer between the fresh-gas mix and the recirculating exhaust gas allows the combustion instabilities to be controlled. One possibility in this respect is acoustic excitation, as known from EP-A1 0 918 152.
Fuel staging allows the flame position to be influenced and therefore the influence of flow instabilities and time delay effects to be reduced (as described for example in EP-A1 0 999 367).
A further mechanism which can give rise to thermoacoustic oscillations is fluctuations in the mixing ratio between fuel and air.
The document WO-A1-01/96785 relates to a burner consisting of a torsion generator for a combustion air current, a torsion chamber, and means of introducing fuel to the combustion air current, whereby the torsion generator exhibits entrance openings to admit air for the combustion air current, which enters the torsion chamber tangentially, and the means for introducing fuel to the combustion air current comprise at least an initial fuel intake with an initial group of fuel outlet openings arranged substantially in the direction of a burner axis for an initial quantity of premixed fuel. Furthermore, one or more second fuel intake(s), with a second group of fuel outlet openings, arranged substantially in the direction of the burner axis, is/are provided for a second quantity of premixed fuel, whereby the second fuel intake(s) can admit the fuel, independent of the first fuel intake. With the present burner, optimal mixing conditions can be set, even in cases of divers loads, gas qualities, or gas pre-heating temperatures.
The patent application DE-A 1-195 45 310, which was laid open to public inspection, reveals a pre-mixing burner for the purpose of mixing fuel and combustion air prior to ignition, whereby the burner consists, substantially, of at least two partially conical shells, with pertinent partially conical axes and entry channels for the combustion air. The premixing burner is substantially formed of a straight hollow cone, which is delimited by an external conical mantle and an internal conical mantle, in which, in addition, at least two entry channels are arranged tangentially to the inner conical mantle, and along a straight conical mantle line of the conical mantle. The partially conical axes of the partially conical shells formed as a result lie on a common conical axis.